White blister (A. candida; synonyms: A. cruciferum, A. cruciferatum, white rust, staghead) is a plant disease which causes many problems in vegetables crops of cabbage, but also in related species such as rape, mustard and radish. The disease can in principle occur on all cruciferae, so also on wild species such as shepherd's purse (Capsella bursa-pastoris) and wild mustard (charlock mustard, Sinapis arvensis). Contrary to what the name suggests, this is not a rust fungus but an oomycete closely related to downy mildew (Peronospora parasitica) and Phytophtora. Oomycetes are not fungi and, although they also grow in threads, they are more related to algae.
The oomycete causes blisters with spores (sori, pustules) on the leaves, stems and ovaries (siliques) of Brassica plants. Distortions in the form of spots/are also often present. Systemic infection of plants results in abnormal growth, deformations and sometimes sterility of the flowers or inflorescence. The oomycete thrives best at temperatures between and 20° C. and in moist conditions. A leaf wetness period of 2.5 hours is sufficient to result in infection, at which there is an incubation period of 10 to 14 days. Moist weather conditions with moderate temperatures are therefore ideal for infection and spreading of the oomycete.
When spores of A. candida land on a cabbage leaf, they form a germ tube with which they penetrate the leaf. In here, the mycelium grows intercellularly and absorbs nutrients via haustoria. The vegetative spore formation takes place in the zoosporangium which develops under the epidermis. Created herein are the asexual zoospores which, when there is sufficient moisture, are released from the zoosporangia and can then cause new infections. The spores have two whiplash tails (flagellae), one for forward movement and one for the swimming direction.
A. candida can overwinter in the ground in sexual form with thick-walled oospores, which may or may not be on infected plant remnants, or in asexual form (mycelium) on winter-hardened host plants. During mild winters the oomycete does not really go to rest but remains active at a lower level. New plants can be infected in the spring. Plant material can also be already infected on the plant bed without symptoms becoming visible. Spread of the oomycete takes place through sporangia being carried away by air movements, hard rainfall, watering, machines, farm workers and insects, whereby other plants are infected.
Host specialization in A. candida is known and different physiological species and formae specialis are distinguished on the basis of the species or the line which is infected and the aggressiveness of the isolate on the line.
Brassica is a plant genus in the family Brassicaceae (formerly referred to as Cruciferae). The members of this genus are referred to as cabbage or mustard. The genus Brassica comprises a number of important agricultural and horticultural crops, including rape, cauliflower, red cabbage, savoy cabbage, white cabbage, oxheart cabbage, curly cale cabbage, broccoli, Brussels sprouts, Chinese cabbage, turnip cabbage and Portuguese cabbage (tronchuda). Almost all parts of the plants are used as food, such as the roots (turnip), stalks (turnip cabbage), leaves (white cabbage), axillary buds (sprouts), flowers (cauliflower, broccoli) and seeds (rape). Rape and rape seed are also used for oil, both for consumption and for fuel. Some species with white or purple flowers or distinct colour or shape of the leaves are cultivated for ornamental purposes. The Brassica family occurs worldwide and consists of annuals, biennials and perennials. The family also comprises a large number of wild species.
At the moment few agents are known which can be used to control white blister in Brassica. An increasing number of countries in Europe moreover have a policy aimed at reducing the use of crop protection agents. If the use of control agents is no longer allowed at all, this can result in major problems in the cultivation of Brassica species. In crops such as for instance Brassica rapa (syn. campestris) (turnip rape), Brassica juncea (mustard) and Brassica napus (rapeseed) white blister can cause huge losses in yield (Bernier, Can. Plant Dis. Surv. 52: 108, 1972; Fan et al., Can. J. Genet. Cytol. 25: 420-424, 1983); Harper and Pittman, Phytopathology 64: 408-410, 1974; Varshney et al., Theoretical and Applied Genetics 109: 153-159, 2004). In vegetable crops the quality aspect is particularly important. Vegetables such as sprouts, headed cabbage and curly cale cabbage infected by white blister are no longer sellable because of the cosmetic damage. There is therefore a great need for Brassica vegetable crops which are resistant to white blister.
Resistance to white blister is described in diverse Brassica species such as B. rapa, B. napus and B. juncea (Ebrahimi et al., Proc. Am. Phytopathol. Soc. 3: 273, 1976; Delwiche and Williams, Proc. Am. Phytopathol. Soc. 1: 66, 1974; Tiwari et al., Can. J. Of Plant Science 68: 297-300, 1988; Kole et al., Genome 45: 22-27, 2002; Varshney et al., Theoretical and Applied Genetics 109: 153-159, 2004; Tanhuanpad, Theoretical and Applied Genetics 108: 1039-1046, 2004). In addition, partial resistance has been demonstrated in B. oleracea lines (Santos and Dias, Genetic Resources and Crop Evolution 51: 713-722, 2004). Full resistance to white blister in B. oleracea vegetable crops has however not as yet been described.